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Abstract
Progress can be expected in the areas listed below as mechanically compensated zoom lenses continue to dominate the field of infrared zoom lenses. 8.1 Athermalization A variety of approaches to athermalization are currently being utilized. The use of materials such as zinc sulfide and zinc selenide in combination with germanium makes it possible to provide some passive optical athermalization. Hybrid passive∕active mechanical athermalization techniques are also being utilized. The most common arrangement involves the additional movement of lens elements which already move for other reasons, such as zooming and focusing. These movements are either real-time computed or calculated from look-up tables, and the whole process involves carefully selected temperature sensors as part of a closed-loop system. The use of glass substitution during optimization has been demonstrated to be a feasible approach to passive athermalization of infrared zoom lens systems. 8.2 Diffractive Optical Elements Diffractive optical elements are being utilized with increasing frequency in visible and infrared optical systems. They can improve system performance while reducing cost and weight by reducing the number of lens elements. 8.3 Conics and Aspherics The use of conic sections and aspheric surfaces has become more commonplace mainly due to the relative ease of diamond turning these surfaces at low cost to tight tolerances. 8.4 Materials The availability of optical materials in the infrared region of the spectrum continues to be somewhat limited. Some materials, such as gallium arsenide, find greater use in special applications such as laser systems. Zinc selenide is the material of choice in CO2 laser systems due to its low absorption at 10.6 μm.
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CHAPTER 8
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